1. Field of the Invention
The present invention relates to a driving circuit for an active matrix type display device, and more particularly to a scan driving circuit for driving pixel rows in an organic light emitting display device.
2. Discussion of Related Art
In general, an active matrix type display device, such as the organic light emitting display device, includes a pixel array arranged in a matrix pattern at cross over regions between data lines and scan lines.
Here, the scan lines include horizontal lines (i.e., row lines) of a display region (including the pixel array), and sequentially provide a predetermined signal, namely, a scan signal, from a scan driving circuit to the pixel array.
FIG. 1 is a block diagram showing a conventional scan driving circuit. With reference to FIG. 1, the conventional scan driving circuit includes a plurality of stages ST1 to STn, which are serially coupled to a start pulse SP input line. The start pulse SP may also be referred to as a start signal. The plurality of stages ST1 to STn sequentially shift a clock signal C in response to the start pulse SP to generate output signals SO1 to SOn, respectively. Each of second to nth stages ST2 to STn receives and shifts an output signal of a previous stage as a start pulse.
Accordingly, the stages generate output signals SO1 to SOn by sequentially shifting the start pulse SP, and provide the output signals to the pixel array.
FIG. 2 is a circuit diagram of a stage in the scan driving circuit shown in FIG. 1. FIG. 3 is an input/output waveform diagram of the stage shown in FIG. 2. Referring to FIG. 2 and FIG. 3, each stage of a scan driving circuit conventionally uses a master-slave flip-flop. When a clock clk is at low level, such a flip-flop continues to receive an input and maintains a previous output.
In contrast, when the clock clk is at high level, the flip-flop maintains an input signal In received when the clock clk is at the low level and outputs it as an output signal Out1, but no longer receives the input signal In.
In the aforementioned circuit, an inverter included in the flip-flop has a problem in that a static current flows when an input In to the inverter is at low level. Furthermore, in the flip-flop, the number of inverters receiving a high-level input is the same as that of inverters receiving a low-level input. Accordingly, the static current flows through one half of all the inverters in the flip-flop, thereby causing increased power consumption.
An inset in FIG. 2 shows a more detailed circuit for the inverter. A voltage value corresponding to a ratio of resistances connected between a power supply VDD (e.g., a first voltage source) and a ground GND (e.g., a second voltage source) determines a high level of an output voltage out of the inverter including transistors M1′ and M2′. A low level of the output voltage out is set to be greater than the voltage level of the ground GND by a threshold voltage Vth of the transistor M2′ used in the inverter circuit.
Due to characteristic deviations of the transistors, since levels of an input voltage is different according to the respective stage, in the case where the circuit of FIG. 2 is used, deviation occurs when the output voltage is at high level, with the result that the circuit may be erroneously operated.
Moreover, the deviation in the low level of the output voltage causes a deviation in on-resistance of an input transistor of an inverter included in the circuit of FIG. 2 to occur, thereby impacting a deviation in a high level of the output voltage. In particular, since a display panel of an organic light emitting display device uses a transistor having a large characteristic deviation, such a problem is more serious.
Further, in the inverter, an electric current flows through the input transistor to charge an output terminal, whereas the electric current flows through a load transistor to discharge the output terminal. Upon charging of the output terminal, a source-gate voltage of the load transistor is gradually reduced, and accordingly a discharge current is rapidly reduced. This causes the discharge efficiency to be deteriorated.